Find Prime Number - find prime number code

dedyradix

Mar 28 2007, 01:27 PM

CODE

Public Function GetPrimeNo (InNoWanted As Integer) As Long 

Dim lPrimes() As Long 
Dim lTrial As Long 
Dim lCount As Integer 
Dim lFound As Integer 
Dim lCounter As Integer 

ReDim lPrimes(InNoWanted)

lTrial = 5 
lCount = 3 
lFound = 0 
'Define the first three prime number 
' > lPrimes(0) = 2 
lPrimes(1) = 3 
lPrimes(2) = 5 
Do 
lTrial = lTrial + 2 
lFound = 0 
For lCounter = 0 To lCount - 1 
lFound = (lTrial Mod lPrimes(lCounter)) = 0 
If lFound Then 
Exit For 
End If 

lCounter = lCounter + 1 
Next lCounter 

If lFound = 0 Then 
lPrimes(lCount) = lTrial 
lCount = lCount + 1 
End If 

Loop While (lCount < InNoWanted) 

GetPrimeNo = lPrimes(InNoWanted - 1) 
End Function

Reply

ghostrider

Mar 28 2007, 08:04 PM

Interesting function. What exactly is the purpose of the argument InNoWanted?

Reply

Galahad

Apr 4 2007, 11:31 AM

That's great function... I'm really no good at math, and I can emember that on one or two occasions I needed such a function, to find a prime number... I'll keep this in my code base, for future reference... Great work

Reply

hitmanblood

May 9 2007, 09:04 PM

Are you sure that this works as I checked it at least I scanned it and it seems like it will return an error because of for loop. I am quite new to the visual basic but I am still learning that is in the process of learning. Maybe I am just wrong when I have more time I'll check it.

Reply

ghostrider

May 9 2007, 11:08 PM

I am reviewing your function once and you have a couple mistakes so far. I've been using VB for about 11 years now, and I will point them out to you, and post a bug free function.

You didn't post any information about how your function works. From what I can gather, the argument InNoWanted, will return the nth prime number, where n is the value of InNoWanted. For example, your function should produce the following results.

--------------------------------------
| InNoWanted | Value Returned |
--------------------------------------
| 1 | 2 |
--------------------------------------
| 2 | 3 |
--------------------------------------
| 3 | 5 |
--------------------------------------
| 4 | 7 |
--------------------------------------

The first error I have encountered is this that a value of zero will mess stuff up. You should add a check to make sure a value of zero is not entered:

CODE

Public Function GetPrimeNo(InNoWanted As Integer) As Long

Dim lPrimes() As Long
Dim lTrial As Long
Dim lCount As Integer
Dim lFound As Integer
Dim lCounter As Integer

    If InNoWanted = 0 Then
    Exit Function
    End If

Nextly, it returns an error because you do not have enough elements in your array. You define 3 below this ReDim statement. Therefore you should add 3 to the amount of elements to create.

CODE

ReDim lPrimes(InNoWanted + 3)

You also commented out some code. You need that to have your program run correctly.

CODE

'Define the first three prime number
lPrimes(0) = 2
lPrimes(1) = 3
lPrimes(2) = 5

Other than that, you have no other programming mistakes in your code. Below is the fixed version:

CODE

Public Function GetPrimeNo(InNoWanted As Integer) As Long

Dim lPrimes() As Long
Dim lTrial As Long
Dim lCount As Integer
Dim lFound As Integer
Dim lCounter As Integer

    If InNoWanted = 0 Then
    Exit Function
    End If

ReDim lPrimes(InNoWanted + 3)

lTrial = 5
lCount = 3
lFound = 0
'Define the first three prime number
lPrimes(0) = 2
lPrimes(1) = 3
lPrimes(2) = 5
Do
lTrial = lTrial + 2
lFound = 0
For lCounter = 0 To lCount - 1
lFound = (lTrial Mod lPrimes(lCounter)) = 0
If lFound Then
Exit For
End If

lCounter = lCounter + 1
Next lCounter

If lFound = 0 Then
lPrimes(lCount) = lTrial
lCount = lCount + 1
End If

Loop While (lCount < InNoWanted)

GetPrimeNo = lPrimes(InNoWanted - 1)
End Function

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